1. Field of the Invention
The present invention relates to a fusing system and a temperature control method thereof for use in an image forming apparatus such as a laser beam printer, a facsimile machine (FAX), a copier, and the like, and more particularly, it relates to a fusing system and a temperature control method thereof for use in an image forming apparatus, which is capable of stably fusing a toner image onto a recording medium such as a sheet of printing paper, by minimizing a fluctuation width in surface temperature of a fusing roller in the fusing system.
2. Description of the Prior Art
A general image forming apparatus such as a copier, a laser beam printer and the like, prints a desired image on a recording medium i.e. a sheet of printing paper using a series of image forming processes. The image forming processes include electrically charging a surface of a photosensitive drum by rotating an electrostatic charging roller disposed adjacent to the photosensitive drum, exposing the surface of the photosensitive drum to a laser beam projecting from a laser scanning unit (LSU) to thus form an electrostatic latent image on the surface of the photosensitive drum, developing the electrostatic latent image formed on the surface of the photosensitive drum into a toner image having a powdery state i.e. a visible image by applying a toner to the surface of the photosensitive drum, transferring the toner image formed on the surface of the photosensitive drum onto the sheet of printing paper which passes between the photosensitive drum and a transfer roller which are in contact with each other under a predetermined pressure, supplying a predetermined transfer voltage to the transfer roller and the photosensitive drum, and fusing the toner image of the powdery state to affix it onto the sheet of printing paper by heating the sheet of printing paper with the toner image transferred thereon through a fusing system including a fusing roller.
Generally, in the process of fusing the toner image, a halogen lamp is employed as a heating source for the fusing system. The halogen lamp is disposed inside the fusing roller and/or a fusing backup roller to heat a surface of the fusing roller and/or the fusing backup roller to a predetermined temperature using a radiant heat thereof.
FIG. 1 is a block diagram illustrating an example of a fusing system 10 of a general electrophotographic image forming apparatus.
The fusing system 10 includes a cylinder-shaped fusing roller 11, and a halogen lamp 12 disposed in an inner center thereof. The halogen lamp 12 generates heat inside the fusing roller 11,.
Disposed under the fusing roller 11 is a fusing backup roller 13. As shown in FIG. 3, the fusing backup roller 13 is resiliently supported by a spring 13a, to enable the pressing of a sheet of printing paper 14 passing between the fusing roller 11 and the fusing-backup roller 13, toward the fusing roller 11 with a predetermined pressure.
Accordingly, while the sheet of printing paper 14 is passing between the fusing roller 11 and the fusing backup roller 13, a toner image 14a formed on the sheet of printing paper 14 in the powdery state is subjected to a predetermined pressure and a predetermined heat. As a result, the toner image 14a is fused and fixed onto the sheet of printing paper 14 by the predetermined pressure and the predetermined heat imposed thereto via the fusing roller 11 and the fusing-backup roller 13.
Referring to FIG. 1, installed at a side of the fusing roller 11 are a thermistor 15, a thermostat 16 and a power switching part 19 such as a thyristor. The thermistor 15 is for detecting or sensing a surface temperature of the fusing roller 11 via an electric signal, the thermostat 16 is for blocking an electric power to the halogen lamp 12 when the surface temperature of the fusing roller 11 exceeds a given threshold, and the power switching part 19 is for switching a power supply of an AC power source 18 to the halogen lamp 12 according to a signal from a controller 20.
The thermistor 15 detects the surface temperature of the fusing roller 11, and transmits the detected temperature to the controller 20. The controller 20 compares the detected temperature with a predetermined set temperature, and regulates the power supply to the halogen lamp 12 using the power switching part 19, thereby maintaining the surface temperature of the fusing roller 11 at a fusing temperature suitable to fuse the toner image 14a and to affix it onto the sheet of printing paper 14.
As shown in FIG. 4, the controller 20 usually controls the surface temperature of the fusing roller 11 using a temperature control process that includes an initial heating step S1 of heating the surface temperature of the fusing roller 11 to a print standby temperature, a print standby step S2 of maintaining the surface temperature of the fusing roller 11 at the print standby temperature and waiting for a print command, and a printing step S3, S4, S5 and S6 of maintaining the surface temperature of the fusing roller 11 higher than the print standby temperature to offset a loss in heat which occurs during the fusing operation of fusing the toner image.
At each of the steps S1 through S6 of the temperature control process, the controller 20 controls the power supply to the halogen lamp 12, by comparing the detected surface temperature of the fusing roller 11 with the set temperature and then turning on the halogen lamp 12 through the power switching part 19 when the detected temperature is below the set temperature and turning off the halogen lamp 12 when the detected temperature is above the set temperature, and thereby the surface temperature of the fusing roller 11 is maintained within a given range.
Also, the thermostat 16 functions as an overheating prevention means to protect the fusing roller 11 and its neighboring components, in case the thermistor 15 and the controller 20 fail to regulate the surface temperature of the fusing roller 11.
In such a conventional fusing system 10, as shown in FIG. 2, the fusing roller 11 is usually comprised of an aluminum cylinder having a rubber layer 11a of low thermal conductivity coated on an outer surface thereof.
The rubber layer 11a functions to make the sheet of printing paper 14 maintain a given contact area with the fusing roller 11 while it passes through a nip between the fusing roller 11 and the fusing backup roller 13, thereby providing enough time to supply the heat radiated from the halogen lamp 12 to the sheet of printing paper 14, and at the same time, keep the heat radiated from the halogen lamp 12, thereby preventing the surface temperature of the fusing roller 11 from suddenly lowering even though the sheet of printing paper 14 passes therethrough. However, due to low thermal conductivity, the rubber layer 11a presents a problem of increasing the time required for the heat supplied from the halogen lamp 12 to reach the surface of the fusing roller 11.
More specifically, in the process of fusing the toner image, when the halogen lamp 12 is turned off after the thermistor 15 detects that a surface temperature of the rubber layer 11a of the fusing roller 11 has reached the fusing temperature, the surface temperature of the rubber layer 11a continues to increase above the fusing temperature for a given period due to the heat of the aluminum cylinder of the fusing roller 11 heated by the halogen lamp 12 to reach fusing temperature.
Also, when the halogen lamp 12 is turned on after the thermistor 15 detects that the surface temperature of the rubber layer 11a of the fusing roller 11 has fallen below the fusing temperature, the surface temperature of the rubber layer 11a further drops for a given time until the temperature of the aluminum cylinder rises to a certain temperature level able to increase the surface temperature of the rubber layer 11a to the fusing temperature. Since the temperature of the aluminum cylinder goes below the certain temperature that is able to increase the surface temperature of the rubber layer 11a to the fusing temperature, for a given time the surface temperature of the rubber layer 11a rises above the fusing temperature due to the heat of the aluminum cylinder of the fusing roller 11.
For example, as shown in FIG. 5, when the halogen lamp 12 is driven for 90 seconds and then turned off after the surface temperature of the rubber layer 11a of the fusing roller 11 has reached the fusing temperature, for example, 180° C., the surface temperature of the rubber layer 11a further rises above the fusing temperature, since the aluminum cylinder of the fusing roller 11 has been heated to a temperature of 230° C. Whereas, when the halogen lamp 12 is turned on after the surface temperature of the rubber layer 11a of the fusing roller 11 has risen above the fusing temperature due to the temperature of the aluminum cylinder of the fusing roller 11 and then fallen again below the fusing temperature, the surface temperature of the rubber layer 11a further falls for a given time until the heat of the aluminum cylinder of the fusing roller 11 heated by the halogen lamp 12 reaches the surface of the rubber layer 11a. 
Thus, the fusing roller 11 having the rubber layer 11a does not suddenly change the surface temperature thereof as compared with a fusing roller formed of only an aluminum cylinder but, instead, presents a problem of increasing the fluctuation width in the surface temperature thereof.
If the fluctuation width in the surface temperature of the fusing roller 11 increases, the fusing temperature is unstably regulated. Thereby, when the sheet of printing paper passes through the fusing roller, the toner image formed on the sheet of printing paper is irregularly fused and affixed.
Accordingly, in order to solve the problem of the fusing roller 11 having the rubber layer 11a, a new fusing system and a temperature control method, which does not simply turn on or off the halogen lamp 12 at predetermined or non-predetermined intervals of time by using the thermistor 15, as in the conventional fusing system 10. The new fusing system and temperature control method should regulate the surface temperature of the fusing roller 11 into the fusing temperature, by taking into account the fluctuation difference in the surface temperature of the rubber layer 11a. 